The present invention relates to "transflective" optical filters, i.e. filters that are suitable for being observed in transmissive mode when backlighted by a source of light placed on the opposite side to the observer, and in reflective mode when lighted via the front face, e.g. by ambient light.
A particulary important, but not exclusive, application of the invention lies in making automobile dashboard panels. By day, the panel is not backlighted. Marking (dials for a revolution counter, a speedometer a fuel gauge) must appear to the driver as bright marks on a dark background, generally white on black. Alarm indicators should be difficult to see or invisible so long as the panel is not backlighted at their locations. At night, the panel is backlighted and the markings should appear in coloured form, frequently green or orange. When an indicator is activated, it should also appear coloured on a black background, and the colour is frequently red for an alarm indicator.
At present, such panels are generally made by multi-pass silkscreen printing. To do this, a sheet of translucent plastics material is used on which the various coloured layers are printed in succession (red, green and yellow layers, in general, when it is desired to have a complete palette of colours available). Thereafter, a non-coloured diffusing layer is formed by silkscreen printing. Finally, a mask is deposited by silkscreen printing, with the markings remaining transparent while the remaining surfaces are opaque.
The limitations inherent to multi-pass silkscreen printing mean that it is then necessary to use at least seven silkscreen passes. The limit resolution possible with this technology is of the order of 100 microns.
A method is also known of manufacturing a multicoloured filter array for a liquid crystal display (document EP 341348). That array is made up of two portions. The first portion constitutes a polychromatic "slice" that is exposed on a backing and is then peeled apart. The second portion is made separately and constitutes a black contour line pattern. Thereafter, the first portion is fixed on the second portion.